A proton exchange membrane (PEM) fuel cell is typically composed of a seven-layered structure, including (a) a central PEM electrolyte layer for proton transport; (b) two electro-catalyst layers on the two opposite primary surfaces of the electrolyte membrane; (c) two fuel or gas diffusion electrodes (GDEs, hereinafter also referred to as diffusers) or backing layers stacked on the corresponding electro-catalyst layers (each GDE comprising porous carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell); and (d) two flow field plates (or a bi-polar plate) stacked on the GDEs. The flow field plates are typically made of graphite, metal, or conducting composite materials, which also serve as current collectors. Gas-guiding channels are defined on a GDE facing a flow field plate or, more typically, on a flow field plate surface facing a GDE. Reactants (e.g., H2 or methanol solution) and reaction products (e.g., CO2 at the anode of a direct methanol fuel cell, and water at the cathode side) are guided to flow into or out of the cell through the flow field plates. The configuration mentioned above forms a basic fuel cell unit. Conventionally, a fuel cell stack comprises a number of basic fuel cell units that are electrically connected in series to provide a desired output voltage. If desired, cooling channels and humidifying plates may be added to assist in the operation of a fuel cell stack.
In one common practice, a fuel flow field plate and an oxidant gas flow field plate are separately made and then assembled together to form a bipolar plate (one side of a bipolar plate serving as a negative terminal and the other side as a positive terminal, hence the name). In some cases, an additional separator is sandwiched between the two flow field plates to form a three-layer bipolar plate. It would be highly advantageous if the flow filed plates and the separator can be mass-produced into an integrated bipolar plate assembly. This could significantly reduce the overall fuel cell production costs and reduce contact ohmic losses across constituent plate interfaces. The bipolar plate is known to significantly impact the performance, durability, and cost of a fuel cell system. The bipolar plate, which is typically machined from graphite, is one of the most costly components in a PEM fuel cell.
Fluid flow field plates or bipolar plates have open-faced channels formed in one or both opposing major surfaces for distributing reactants to the gas diffuser plates which are the anode and cathode backing layers, typically made of carbon paper or fabric. The open-faced channels also provide passages for the removal of reaction products and depleted reactant streams. Optionally, a bipolar plate may have coolant channels to manage the fuel cell temperature. A flow field plate or bipolar plate should have the following desirable characteristics: high electrical conductivity (e.g., preferably having a conductivity no less than 100 S/cm), low permeability to fuel or oxidant fluids, good corrosion resistance, and good structural integrity.
Conventional methods of fabricating fluid flow field plates or bipolar plates require the engraving or milling of flow channels into the surface of rigid plates formed of a metal, graphite, or carbon-resin composite. These methods of fabrication place significant restrictions on the minimum achievable fuel cell thickness due to the machining process, plate permeability, and required mechanical properties. Further, such plates are expensive due to high machining costs. The machining of channels into the graphite plate surfaces causes significant tool wear and requires significant processing times.
Alternatively, fluid flow field plates can be made by a lamination process (e.g., U.S. Pat. No. 5,300,370, issued Apr. 5, 1994 to Washington, et al.), wherein an electrically conductive, fluid impermeable separator layer and an electrically conductive stencil layer are consolidated to form one open-faced channel. Presumably, two conductive stencil layers and one separator layer may be laminated to form a three-layer bipolar plate. Such laminated fluid flow field assemblies tend to have higher manufacturing costs than integrated plates, due to the number of manufacturing steps associated with forming and consolidating the separate layers. They are also prone to delamination due to poor interfacial adhesion and vastly different coefficients of thermal expansion between a stencil layer (typically a metal) and a separator layer.
A variety of composite bipolar plates have been developed, which are mostly made by compression molding of polymer matrices (thermoplastic or thermoset resins) filled with conductive particles such as graphite powders or fibers. Because most polymers have extremely low electronic conductivity, excessive conductive fillers have to be incorporated, resulting in an extremely high viscosity of the filled polymer melt or liquid resin and, hence, making it very difficult to process. Bi-polar plates for use in PEM fuel cells constructed of graphite powder/fiber filled resin composite materials and having gas flow channels are reviewed by Wilson, et al (U.S. Pat. No. 6,248,467, Jun. 19, 2001). Injection-molded composite-based bipolar plates are disclosed by Saito, et al. (U.S. Pat. No. 6,881,512, Apr. 19, 2005 and No. 6,939,638, Sep. 6, 2005). These thermoplastic or thermoset composites exhibit a bulk conductivity significantly lower than 100 S/cm (the US Department of Energy target value), typically not much higher than 10 S/cm.
Besmann, et al. disclosed a carbon/carbon composite-based bipolar plate (U.S. Pat. No. 6,171,720 (Jan. 9, 2001) and No. 6,037,073 (Mar. 14, 2000)). The manufacture process consists of multiple steps, including production of a carbon fiber/phenolic resin preform via slurry molding, followed by a compression-molding step. The molded part is then pyrolyzed at a high temperature (1,500° C.-2,500° C.) to obtain a highly porous carbon/carbon composite. This is followed by chemical vapor infiltration (CVI) of a carbon matrix into this porous structure. It is well-known that CVI is a very time-consuming and energy-intensive process and the resulting carbon/carbon composite, although exhibiting a high electrical conductivity, is very expensive.
Instead of using pyrolyzation and CVI to produce carbon/carbon composites, Huang, et al. (US Patent Application Pub. No. 2004/0229993, Nov. 18, 2004) discloses a process to produce a thermoplastic composite with a high graphite loading. First, polymer fibers, such as thermotropic liquid crystalline polymers or polyester, reinforcing fibers such as glass fibers, and graphite particles are combined with water to form a slurry. The slurry is pumped and deposited onto a sieve screen. The sieve screen serves the function of separating the water from the mixture of polymer fibers, glass fibers and graphite. The mixture forms a wet-lay sheet which is placed in an oven. Upon heating to a temperature sufficient to melt the polymer fibers, the wet-lay sheet is allowed to cool and have the polymer material solidify. Upon solidification, the wet-lay sheet takes the form of a sheet material with reinforcement glass fibers held together by globules of thermoplastic material, and graphite particles adhered to the sheet material by the thermoplastic material. Several of these sheets are then stacked, preferably with additional graphite powder interspersed between sheets, and compression-molded in a hot press. After application of heat and pressure in the press, one or more formed bipolar plates are obtained, where the bipolar plates are a composite of glass fibers, thermoplastic matrix and graphite particles. There are several drawbacks associated with this composite composition and method:
(1) The fabrication process is tedious, consisting of many manual operations, and is not readily amenable to mass production.
(2) The composition requires heating the mixture above the melting point of the thermoplastic material twice—(a) the first time being to melt out the thermoplastic solid, allowing the melt to flow to the contact points between reinforcement fibers so as to bond the fibers together when the thermoplastic is cooled and (b) the second time to melt the thermoplastic so as to wet the remaining reinforcement fibers and graphite powders and form the matrix of a structural composite plate when the thermoplastic solidifies. Since engineering thermoplastics typically have a high melting point (e.g., >220° C. for polyester), it would take some time to heat up to that temperature and take some time to cool it down. The cycle times are long and the process is energy-intensive.(3) With this process, it appears difficult to achieve a graphite proportion above 50% (and, hence, conductivity above 100 S/cm) without interspersing additional graphite powder between layers of stacked preform sheets (an operation called “dry-lay”) prior to compression-molding. This is evidenced by FIG. 2 of Huang's application, which indicates that all samples with the resulting conductivity greater than 100 S/cm were prepared by a combined wet-lay (slurry molding) and dry-lay procedure. Such labor-dependent operations make the whole process time-consuming and labor-intensive. Dry-laid graphite powder between layers, although imparting high electrical conductivity to the composite, tend to form graphite-rich interfacial layers which are brittle and weak and tend to compromise the mechanical integrity of the resulting composite laminate.
The flow field plate or bipolar plate should be constructed from inexpensive starting materials, materials that are easily formed into any plate configuration, preferably using a continuous molding process, and materials that are corrosion resistant in low temperature fuel cells and that do not require further processing such as high temperature pyrolyzation treatments. Any laminated or multi-layer plate should have adequate bonding between layers to ensure structural integrity and reduced contact resistance (reduced power loss due to joule heating).
Accordingly, a primary object of the present invention is to provide a highly conductive composite composition and a fuel cell flow field plate or bipolar plate from this composition that can be made with a continuous process, which is suitable for mass production. The resulting fuel cell component is highly conductive and, hence, can be used as a current collector in a fuel cell with reduced contact resistance.
Another object of the present invention is to provide a highly conductive composite material for fuel cell bipolar plates which can be made without involving high temperature treatments.
Still another object of the present invention is to provide a highly conductive composite material for fuel cell bipolar plates which can be made without involving melting and cooling a thermoplastic twice.
Another object of the present invention is to provide a highly conductive composite material for fuel cell bipolar plates which is based on a thermoset resin that can be molded with a fast cycle.
Another object of the present invention is to provide a process for continuously producing a highly conductive composite-based flow field plate or bipolar plate.